Mesa is a 3-D graphics library with an API which is very similar to that of OpenGL. To the extent that Mesa utilizes the OpenGL command syntax or state machine, it is being used with authorization from SGI.
While Mesa is not a licensed OpenGL implementation, it is currently being tested with the OpenGL conformance tests. Despite these technical/legal terms, you may find Mesa to be a valid alternative to OpenGL. Most applications written for OpenGL can use Mesa instead without changing the source code.